The suspect is described as being in his early-to-mid 20s, about 6ft in height, slim build and with dark hair brushed to the side.

He was wearing a cream jumper, shirt, tie, grey dress trousers and dark shoes and was carrying a large wallet.

Detective Constable Paul Farquhar, the British Transport Police officer in charge of the inquiry, said: “This can only be described as a cowardly attack and there were others travelling on the train who would have witnessed the assault. I am urging anyone who has information to get in touch with BTP as a matter of urgency.”